To ensure the safety of the section of the track between your marshal's point and the marshal's point beyond yours, i.e. looking down the track in the direction that the riders travel.  Always ensure that you can see the next marshal's point.  This means that the entire track is marshaled.

You should never leave your marshal's point to assist a fallen rider at the risk of approaching riders not seeing you.

In the event of a rider falling, the YELLOW flag must be raised in a motionless manner to warn oncoming riders to slow down and take care. 

If the incident is of a more serious nature (i.e. a rider in the track, in what you consider to be a dangerous position) the YELLOW flag must be waved with urgency. To warn riders that they must slow down and be prepared to stop. Strictly no overtaking

If the rider appears to be injured hold your first aid flag (white /red  cross) in the straight up position to signal the first aid personnel, and continue to wave the yellow flag. UNDER NO CIRCUMSTANCES LEAVE YOUR POINT TO ATTEND TO AN INJURED RIDER; you are the only warning to other riders that a rider is injured and on the track.

Any competitor that ignores your flag signals should be reported to the Clerk of the Course immediately, so that the appropriate action can be taken against the rider concerned.

You must never leave your marshal's point until the race is finished, or has been stopped.  REMEMBER that the ONLY person empowered to stop the race is the Clerk of the Course.